Why are you wanting to split them? Too large? Want a smaller dose?

Have you considered the syrups? Plain Robitussin only has guaifenesin
(same as what is in the plain Mucinex, and by far the most readily
available expectorant).

Adams (maker of Mucinex) has patent protection for OTC extended-release
guaifenesin. So, if you're looking for extended-release guaifenesin,
Mucinex is the only one. The label says not to crush/chew/break, but
splitting them shouldn't create any serious issues if you're only taking
half of a tablet since the tablets don't rely on a film coating for
extended release. The extended-release properties will be diminished
since the half-tablet will dissolve more rapidly than whole
tablets. They may not split well though. They may end up crumbling on you;
I have never tried.

If you were splitting Mucinex and then still taking a full dose, the
tablets will dissolve quicker, thereby shortening the extended-release
properties as well as giving a higher peak concentration and greater
potential for adverse reactions.

There are non-extended-release guaifenesin tablets available where
splitting wouldn't cause any issues. They are typically store-brand type
products, so the product name depends upon which pharmacy/store you use
(eg. Walgreens' Mucus Relief).